Measurement sensors are however imperfect and exhibit intrinsic errors or measurement bias, which may moreover vary during navigation. Furthermore, they are subject to measurement noise, in the sense that random variations not corresponding to the variations in the measured quantity get superimposed on the useful signal, representing the physical quantity sought. The electrical measurement signals are furthermore processed by electronic circuits which themselves introduce noise.
The bias and measurement noise are all the more detrimental since the position computations made on the basis of the measurement results from the sensors involve integrations. The integration gives rise to a drift in the measured value, which drift increases progressively over time when the integrated value is biased at the start. Double integration (integral of acceleration to give the speed followed by integral of speed to give the position) further increases this drift in considerable proportions.
To summarize, inertial platforms are very accurate over a very short duration but are subject, owing to the systematic time integration of the biases, to a sizeable drift which makes it necessary to reset them periodically on the basis of other position information (or speed information).
A mode of resetting that has long been used has consisted in using a network of ground beacons to provide aircraft overflying them with position signals making it possible to reset their navigation instruments.
More recently, inertial platforms have begun to be reset on the basis of satellite based positioning receivers carried onboard aircraft and deriving position and speed information, in a terrestrial reference frame, from signals that they receive from the satellites.
Hybrid platforms are thus produced that profit both from the excellent very short term measurement quality of inertial platforms (measurement hardly affected by noise) and from the high geographic position accuracy offered by satellite based positioning systems.
However, the position measurements made by satellite based positioning receivers are intrinsically greatly affected by noise in the short term, so that their measurements would have to be averaged in order to determine an accurate position; however, in a mobile vehicle, and especially an airplane moving rapidly, it is not possible to wait to average the measurements in order to obtain an accurate position since the airplane will have moved between two measurements.
Other problems may arise, such as changes of constellation of satellites observed by the receivers, that give rise to abrupt jumps in measured position, or faults with satellites that emit erroneous signals, or even faults with the inertial platform.